The article is devoted to the diagnosis of bearing units working with hydrocarbon mixtures. The diagnostic is based on the photometric method. This method allows you to determine the internal damage to the bearing unit at the stage of the appearance of the microcrack. The photometric method is based on changing the color of the indicator fabric. As the indicator substance, a bromine solution was used which had a dark brown color. If there is a leak, the hydrocarbon reacts with bromine molecules and a bright spot appears on the tape. Indicator fabric is applied to the bearing unit during assembling.The material of the article contains a model of the physicochemical process, which is the basis of the photometric method. Various stages of leakage of hydrocarbon mixtures through microcracks were studied. The results of the experimental studies are consistent with the theoretical propositions. The proposed diagnostic method can be used as a method for non-destructive testing of bearing units.